The present invention relates to the extension of the shelf life of packaged, prepared foods, such as meat-and broiler patties, smoked fish, mayonnaise based vegetable salads, as well as sausages, bread, and egg-butter by reducing the oxygen content with an enzymatic method.
The invention also relates to a method for extending the shelf-life of foods, whereby an enzymatic reaction creates an environment within the foodstuff that is both microbicidal and microbistatic.
The invention further relates to the use of an enzymatic method to reduce the population of pathogenic and food-spoilage microorganisms within a food product.
The invention also relates to the use of this enzymatic method alone or together with a modified atmosphere packaging technology.
The shelf life of a food product is most often dependent on the rate of microbiological spoilage. Both the characteristics of the food product itself, such as water activity, pH, redox potential, antimicrobial agents, chemical and biological composition of the product, as well as the environment in which it is stored (temperature, humidity and the gas composition of the package) effect the rate of spoilage.
In some cases preservatives can be added to prohibit both the growth of microorganisms and the production of toxins harmful to humans. The most widely used preservatives are benzoic acid and sodium benzoate, ethyl-, methyl- and propylparabens, sorbic acid and calcium, sodium and potassium sorbates, as well as propionic acid and calcium, sodium and potassium propionates.
Benzoic acid and sodium-benzoate can be used in products with low pH values, the greatest activity being at pH values below 4.5. At neutral pH values these compounds are essentially ineffective. This restricts the use of benzoic acid and sodium benzoate to the high-acid products such as salad dressings, soft drinks and tomato catsup. The problem with soft drinks is that the benzoates may impart a disagreeable taste at very low levels.
Ethyl-, methyl- and propylparabens are less sensitive to pH than benZoic acid and benzoates. The use of these preservatives in most foods is, however, strictly prohibited by law in many countries.
Sorbic acid and sorbates are the most widely used preservatives. Their antimicrobial activity is also dependent on the pH value but unlike benzoic acid and benzoates they still work at pH values 6.0-6.5. The use of sorbates as food preservatives is limited by law to particular food products. In many countries, it is not allowed in prepared foods such as those described in the present invention.
Propionic acid and propionates are mainly used in bread and other grain products to prevent molding. In general, most countries limit the use of propionates to these types of products.
Although many of the above mentioned chemical preservatives are effective in preventing microbial growth and therefore extending the shelf-life of food products in which they are used, increased awareness of the health risks associated with such preservatives is currently leading to greater restrictions on their use. Commonly used preservatives such as sulfites have been found to cause allergic reactions in unsuspecting consumers. Even in cases where a preservative has been found to be safe, "health conscious" consumers and consequently food producers have sought to find more "natural" methods of storing foods.
One such method is by the removal of oxygen. Because many of the microorganisms that are responsible for the spoilage of food require oxygen to grow, their growth in food products can be restricted or, in some cases, completely halted by the elimination of oxygen from the environment in which they are stored.
Oxygen removal has been suggested for the purpose of minimizing detrimental oxidative processes in food. For example, processed foods have been packaged either in a vacuum or in a modified atmosphere. It is also possible to employ oxygen absorbing agents such as iron powder, (Prepared Foods 3:91-98 (1988) or sulfites or ascorbic acid (Food Technology 40:94-97, 100-102 (1988) Within the food package. However, several problems are attendant with such oxygen removal methods.
The level of injurious free oxygen is reduced in vacuum-packaging technology, thus decreasing the rates of microbiologal spoilage and oxidative reactions. The problems with vacuum-packing are that prepared foods with round shape tend to flatten, slices stick together and liquid frequently is drawn out of the product. Fresh meat also looses its color when vacuum-packaged.
In modified atmosphere packaging, the shelf life of a product can be extended over that in air. The suitable gas composition depends on the product. The most common gas compositions are 20% CO.sub.2 +80% O.sub.2, 20% CO.sub.2 +80% N.sub.2, 100% N.sub.2, and different combinations of the three gases O.sub.2, CO.sub.2 and N.sub.2. The problem with modified packaging technology is that great amounts of carbon dioxide cause discoloration and packages shrink when CO.sub.2 is absorbed by the product. In some cases, the CO.sub.2 can cause off-odors when the package is opened, and CO.sub.2 can have an off-flavor. The amount of residual oxygen is at times sufficient to cause oxidative reactions. In such cases, it can be removed with oxygen absorbers such as iron powder, sulfites, ascorbic acid or glucose oxidase.
The use of oxygen absorbers such as iron powder, ascorbic acid or sulfites are also problematic. The addition of ascorbic acid is regulated by law and it is not always possible to add enough ascorbic acid to prevent all oxidative reactions. Sulfites react with oxygen producing many harmful by-products and sulfites alone can cause off odors and allergic reactions. When using iron powder care must be taken that the product is not commingled with the food.
Enzyme systems have been suggested for the removal of oxygen from packaged foods. Such systems have, however, been wrought with difficulty as the necessary combinations of enzymes, enzyme levels necessary to ensure the creation of the desired conditions, necessary reactants, and other factors necessary to make such enzyme systems workable have not been determined. As a result, these methods have not, to date, been successful for either preserving food products or presenting a consumable product.
Recently, significant production breakthroughs have enabled the production of high yields of purified, highly active glucose oxidase that contains little or no contamination with the enzyme catalase. This has made it possible to develop complex enzyme compositions that solve the foregoing difficulties.
It is therefore, the object of this invention to present an enzymatic method to preserve foodstuffs whereby carefully controlled levels of glucose oxidase and catalase are added to increase the shelf life of food products.
It is also the object of this invention to provide a method whereby glucose oxidase is added to the food product to reduce the oxygen level below 1%, thereby inhibiting the growth of aerobic spoilage-causing microorganisms.
A further object of the invention is to introduce an enzymatic method whereby catalase is introduced to a food product in combination with glucose oxidase to eliminate residual peroxide created by the reaction of glucose oxidase with glucose and oxygen.
The object of the invention is further to provide a method whereby glucose oxidase and glucose are added to a food product along with low levels of catalase, such that the microbicidal effect of the hydrogen peroxide on both food spoilage as well as pathogenic microorganisms can be accomplished prior to its elimination by the catalase.
A further object of the invention is to provide packaging technology wherein residual oxygen is removed from a food package using a glucose oxidase enzyme preparation.